If an aircraft/spacecraft vehicle encounters a failure (such as a jammed control surface or loss of a surface), most controllers cannot adapt to the failure and a crash may occur. In most cases, the vehicle has enough redundant actuation mechanisms to salvage the vehicle. Several airplane crashes have occurred in the past where the pilot is unable to control the damaged airplane due to the pilot's inability to learn to fly this altered aircraft configuration in the very short time available. The flight computer, however, may have the necessary information as well as bandwidth available to learn the new dynamics, and control the vehicle within a reasonable time interval.
The flight computer needs an intelligent controller that flies the vehicle with the baseline controller during nominal conditions, and adapts the design, when the vehicle suffers damage. Thus, given the information about the vehicle from all the available sensors, the control system needs to determine whether the vehicle is in its nominal state or is damaged. One approach to deal with this is to utilize smart algorithms that attempt to identify the vehicle characteristics and to change the control system, if necessary. This approach is known as Indirect Adaptive Control. For systems such as airplanes, there is usually very little time available to make changes to the control system, and this indirect approach is often insufficient to achieve the desired safety metrics. Another approach, known as the direct adaptive control (“DAC”), looks directly at the errors, and updates the control law accordingly. This is typically much faster and meets the timing considerations for airplane system implementations.
The current state of the art implementation consists of the Intelligent Flight Control Architecture that uses a DAC approach. This has been implemented by us at the NASA Ames Research Center, and has been test flown on the F-15 research aircraft at the Dryden Flight Research Center. The update law uses tracking error to change the control law. This approach is based on the work at the Georgia Tech Aerospace Engineering Department, under R. T. Rysdyk and A. J. Calise, “Fault Tolerant Flight Control Via Adaptive . . . Augmentation” AIAA 98-4483.
When operating in the real world, an airplane will always have tracking errors associated with its states. For example, when an pilot provides an aggressive stick command, there is always a large transient tracking error that eventually disappears as the controller continues to perform. Adaptation should typically occur only when the aircraft experiences damage or change in its flight configuration, which the original control design cannot deal with. Usually much effort goes into the design of the nominal baseline control design, which should be changed only if necessary.
What is needed is an approach that implements DAC that looks not just at the tracking error, but rather its characteristics or evolution over time to determine whether the controller needs to be adapted or left alone. The time evolution of the tracking error provides clues for investigating whether the system is in good health or has undergone damage/faults. This crucial piece of available information remains un-utilized in all the existing DAC approaches.